The present invention relates to radiotherapy systems, such as those using ions (such as protons), for the treatment of cancer and, in particular, to a system providing improved treatment speed and accuracy.
External beam radiation therapy may treat a tumor within a patient by directing high-energy radiation in one or more beams toward the tumor. Highly sophisticated external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat a tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated beamlets whose intensities can be controlled so that the combined effect of the beamlets, over the range of angles, allows a complex area to be treated.
X-rays deposit energy along the entire path between the x-ray source and the exit point in the patient. While judicious selection of the angles and intensities of the beamlets of x-ray beamlets can minimize radiation applied to healthy tissue outside of the tumor, the inevitability of x-ray irradiation of healthy tissue along the path to the tumor has led to the investigation of ions, such as protons, as a substitute for x-rays. Unlike x-rays, protons may be controlled to stop within the tissue, eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially to a “Bragg peak” near a point where the proton beamstops within the tissue. The placement of Bragg peaks inside the tumor allows for improved sparing of normal tissue for proton treatments relative to x-ray treatments.
Current proton therapy systems adopt one of two general approaches. In the first approach, termed the “spread out Bragg peak” (SOBP) approach, the range of energies in the proton beam is expand so that their Bragg peaks extend over a range roughly matching the tumor depth. Precise shaping of this volume is provided by a specially constructed correction range compensator which provides additional range shifting to warp the distal edge of the Bragg peaks to the distal edge of the tumor. This treatment approach can treat the entire tumor at once and therefore is fast. But it is difficult to conform the dose to the tumor volume and the construction of a special range compensator is required.
In a second approach, termed the “magnetic spot scanning” (MSS) approach, the proton beam remains narrowly collimated in a “pencil beam” and is steered in angle and range to deposit the dose as a series of small spots within the patient. The spots are located to cover the tumor in successive exposures until an arbitrary tumor volume has been irradiated. This approach is potentially very accurate, but because the tumor is treated in many successive exposures, this approach is much slower than the SOBP approach. Further the small spot sizes create the risk of uneven dose placement or “cold spots” between the treatment spots, something that is exacerbated if there is any patient movement between exposures.